Carbon nanotubes (CNTs), comprising multiple concentric shells and termed multi-wall carbon nanotubes (MWNTs), were discovered by Iijima in 1991 [Iijima, Nature 1991, 354, 56]. Subsequent to this discovery, single-wall carbon nanotubes (SWNTs), comprising single graphene sheets rolled up on themselves to form cylindrical tubes with nanoscale diameters, were synthesized in an arc-discharge process using carbon electrodes doped with transition metals [Iijima, S.; Ichihashi, T. Nature 1993, 363, 603; and Bethune et al. Nature 1993, 363, 605]. These carbon nanotubes (especially SWNTs) posses unique mechanical, electrical, thermal and optical properties, and such properties make them attractive for a wide variety of applications. See Baughman et al., Science, 2002, 297, 787-792.
The diameter and chirality of CNTs are described by integers “n” and “m,” where (n,m) is a vector along a graphene sheet which is conceptually rolled up to form a tube. When |n-m|=3q, where q is an integer, the CNT is a semi-metal (bandgaps on the order of milli eV). When n-m=0, the CNT is a true metal and referred to as an “armchair” nanotube. All other combinations of n-m are semiconducting CNTs with bandgaps in the range of 0.5 to 1.5 eV. See O'Connell et al., Science, 2002, 297, 593. CNT “type,” as used herein, refers to such electronic types described by the (n,m) vector (i.e., metallic, semi-metallic, and semiconducting). CNT “species,” as used herein, refers to CNTs with a particular (n,m) value.
Methods of making CNTs include the following techniques: arc discharge [Ebbesen, Annu. Rev. Mater. Sci. 1994, 24, 235-264]; laser oven [Thess et al., Science 1996, 273, 483-487]; flame synthesis [Vander Wal et al., Chem. Phys. Lett. 2001, 349, 178-184]; and chemical vapor deposition [U.S. Pat. No. 5,374,415], wherein a supported [Hafner et al., Chem. Phys. Lett. 1998, 296, 195-202] or an unsupported [Cheng et al., Chem. Phys. Lett. 1998, 289, 602-610; Nikolaev et al., Chem. Phys. Lett. 1999, 313, 91-97] metal catalyst may also be used.
All known CNT preparative methods lead to polydisperse CNT materials of semiconducting, semimetallic, and metallic electronic types. See M. S. Dresselhaus, G. Dresselhaus, P. C. Eklund, Science of Fullerenes and Carbon Nanotubes, Academic Press, San Diego, 1996; Bronikowski et al., Journal of Vacuum Science & Technology 2001, 19, 1800-1805; R. Saito, G. Dresselhaus, M. S. Dresselhaus, Physical Properties of Carbon Nanotubes, Imperial College Press, London, 1998. Recent advances in the solution phase dispersion [Strano et al., J. Nanosci. and Nanotech., 2003, 3, 81; O'Connell et al., Science, 2002, 297, 593-596] along with spectroscopic identification using bandgap fluorescence [Bachilo et al., Science, 2002, 298, 2361] and Raman spectroscopy [Strano, Nanoletters 2003, 3, 1091] have greatly improved the ability to monitor electrically distinct nanotubes as suspended mixtures and have led to definitive assignments of the optical features of semiconducting [Bachilo et al., Science, 2002, 298, 2361], as well as metallic and semi-metallic species [Strano, Nanoletters, 2003, 3, 1091].
Techniques of chemically functionalizing CNTs have greatly facilitated the ability to manipulate these materials, particularly for SWNTs which tend to assemble into rope-like aggregates [Thess et al., Science, 1996, 273, 483-487]. Such chemical functionalization of CNTs is generally divided into two types: tube end functionalization [Liu et al., Science, 1998, 280, 1253-1256; Chen et al., Science, 1998, 282, 95-98], and sidewall functonalization [PCT publication WO 02/060812 by Tour et al.; Khabashesku et al., Acc. Chem. Res., 2002, 35, 1087-1095; and Holzinger et al., Angew. Chem. Int Ed., 2001, 40, 4002-4005], and can serve to facilitate the debundling and dissolution of such CNTs in various solvents. Scalable chemical strategies have been, and are being, developed to scale up such chemical manipulation [Ying et al., Org. Letters, 2003, 5, 1471-1473, Bahr et al., J. Am. Chem. Soc., 2001, 123, 6536-6542; and Kamaras et al., Science, 2003, 301, 1501].
Carbon nanotube chemistry has been described using a pyramidization angle formalism [S. Niyogi et al., Acc. of Chem. Res., 2002, 35, 1105-1113]. Here, chemical reactivity and kinetic selectivity are related to the extent of s character due to the curvature-induced strain of the sp2-hybridized graphene sheet. Because strain energy per carbon is inversely related to nanotube diameter, this model predicts smaller diameter nanotubes to be the most reactive, with the enthalpy of reaction decreasing as the curvature becomes infinite. While this behavior is most commonly the case, the role of the electronic structure of the nanotubes in determining their reactivity is increasingly important—especially when desiring selectivity among a population of similar-diameter CNTs (such as is often the case with SWNT product). Furthermore, because such structure is highly sensitive to chiral wrapping, chemical doping, charged adsorbates, as well as nanotube diameter, there exists a considerable diversity among these various pathways in addition to a simple diameter dependence, and with implications for separating CNTs by type.
Methods for separating CNTs by electronic type have been reported. See D. Chattopadhyay et al., J. Am. Chem. Soc., 2003, 125, 3370; M. Zheng et al., Science, 2003, 302, 1545-1548; Weisman, Nat. Mater., 2003, 2, 569-570; and commonly assigned, co-pending U.S. patent applications Ser. Nos. 10/379,022 and 10/379,273, both filed Mar. 4, 2003. Additionally, methods for selectively functionalizing CNTs by type have also emerged. See Strano et al., Science, 2003, 301, 1519-1522; L. An et al., J. Am. Chem. Soc., 2004, 126(34), 10520-10521; and commonly assigned, co-pending International Patent Application Serial No. PCT US04/24507, filed Jul. 29, 2004.
While separation of CNTs by type is now a reality, there is still no method for producing large quantities of CNTs having a precisely defined type or range of types (i.e., homogeneous or a particularly-defined range or plurality of types), referred to hereinafter as a “precise population” of CNTs, as all such separation methodologies are carried out on the microscale. In view of the broad range of applications that could potentially benefit from such bulk quantities of CNTs of precise population, a method of “amplifying” the production of such precise populations would be extremely beneficial.